A. Field of Invention
The invention concerns an inverter or an output filter for inverters. The output current of an inverter for higher levels of power generally has a whole series of current harmonics. Those harmonics are troublesome and they occur in an unacceptable fashion in particular when a relatively high level of power has to be fed into a network by way of inverters.
B. Background of the Invention
FIG. 1 shows an inverter with an output choke and a capacitor filter network, which markedly reduces the above-described technical disadvantage of inverters and considerably improves the current quality. The power switches T1 to T6 and the diodes connected in parallel therewith are so switched by means of pulse width modulation of the power switches that three sinusoidal output current phases U, V, W appear at the output of the inverter. The form of the output current is approximated to a sinusoidal oscillation by switching the individual switches T1-T6 on and off in a defined manner. In that case however the above-mentioned harmonics occur due to the inevitable deviations from the reference or target value.
These generated harmonics are in total equal to zero at any moment in the three-phase network. That is the case at any event because there are no other current paths (other electrical circuits). In that situation only symmetrical harmonics occur. The attempt is then made by filter circuitsxe2x80x94as also shown at the output side in FIG. 1xe2x80x94to compensate for those harmonics. As the electrical network, due to the inductive and capacitive sections (components of the transmission line) also have in part very low impedances (resistances) for certain frequencies, undesirably high proportions of the harmonics still flow into the public supply network.
Therefore the object of the invention is to eliminate the above-mentioned disadvantages, so that only current with very low and if possible no harmonics at all is still fed into the public supply network. The invention further seeks to provide that the dc voltage intermediate circuit of the inverter is stabilised in respect of electrical potential (voltage value in relation to the housing, earth or the three-phase system).
In accordance with the invention, to attain that object, there is proposed an inverter having the features set forth in claim 1. Advantageous developments are described in the appendant claims.
With the inverter according to the invention, asymmetrical harmonics are generated at the inverter output side and fed back into the dc intermediate circuit. To produce the asymmetrical harmonics, in addition to the three-phase output choke at the inverter output, there is a further winding (fourth leg). This further winding transmits asymmetrical magnetic fluxes which arise out of the asymmetrical harmonics. The harmonics are collected by way of three resonant circuits, for example three LC-members. If there are still any symmetrical harmonics present at all, they are directly compensated with those resonant circuits. The asymmetrical harmonics (xcexa31xe2x89xa00) are fed back to the dc voltage intermediate circuit. The sum of the asymmetrical harmonics is taken off at the star point of the resonant circuits (of the filter capacitors of the resonant circuits) and coupled in or passed by way of a winding on the fourth leg into the negative bar of the dc voltage intermediate circuit. That gives a closed xe2x80x98harmonic current circuitxe2x80x99 which is illustrated in FIG. 2.
The total current of the asymmetrical harmonics, which is taken off at the star point of the resonant circuits, produces magnetic fluxes in the fourth limb of the output choke. Depending on the current situation, those magnetic fluxes flow back into the three main limbs of the output choke and thus support output inductance. As a result, there is an output inductance which is between about 5% and 20% higher than in a conventional three-phase choke. As only the harmonic current flows in the fourth winding, only very low levels of copper losses occur in this casexe2x80x94in relation to the main coils of the choke.
The invention and advantageous developments as well as the advantages thereof are described by way of example in the following specific description.